Fix the types of builtin prototype objects (#3663)

In ES2015 many builtin prototypes are no longer valid instances of their
respective classes. This change updates affected prototypes to be
regular objects as required.

JerryScript-DCO-1.0-Signed-off-by: Dániel Bátyai dbatyai@inf.u-szeged.hu

tests/jerry/date-construct.js

@@ -13,7 +13,6 @@
 // limitations under the License.
 
 assert (Date.length == 7);
-assert (Object.prototype.toString.call (Date.prototype) === '[object Date]');
 
 var d;
 
@@ -28,8 +27,6 @@ catch (e)
   assert (e.message === "foo");
 }
 
-assert (isNaN(Date.prototype.valueOf.call(Date.prototype)));
-
 d = new Date("abcd");
 assert (isNaN(d.valueOf()));